Hybrid strip-loaded electro-optic polymer/sol-gel modulator

ABSTRACT

A hybrid strip-loaded EO polymer/sol-gel modulator in which the sol-gel core waveguide does not lie below the active EO polymer waveguide increases the higher electric field/optical field overlap factor Γ and reduces inter-electrode separation d thereby lowering the modulator&#39;s half-wave drive voltage Vπ, reducing insertion loss and improving extinction. The strip-loaded modulator comprises an EO polymer layer that eliminates optical scattering caused by sidewall roughness due to etching. Light does not encounter rough edges as it transitions to and from the sol-gel and EO polymer waveguides. This reduces insertion loss.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority under 35 U.S.C. 120 as acontinuation-in-part of U.S. application Ser. No. 12/199,765 entitled“Hybrid Electro-Optic Polymer/Sol-Gel Modulator” filed Aug. 27, 2008 nowU.S. Pat. No. 7,693,355, which claims benefit of priority under 35U.S.C. 119(e) to U.S. Provisional Application No. 60/967,679 entitled“Hybrid Electro-Optic Polymer/Sol-Gel Modulators With Reduced Half-WaveVoltage, Lower Insertion Loss and Improved Contrast” and filed on Sep.6, 2007, the entire contents of which are incorporated by reference.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under NR0000-07-C-0030awarded by the Department of Defense and under DMR0120967 awarded by theNational Science Foundation. The government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electro-optic (EO) modulators and moreparticularly to hybrid EO polymer/sol-gel modulators in which thesol-gel core waveguide does not lie below the active EO polymerwaveguide in the active region.

2. Description of the Related Art

An EO modulator is fundamentally a device that is able to impress anelectrical signal on the amplitude or phase of an optical input throughthe use of special materials that exhibit an EO effect. In suchmaterials, when an electric field is applied to the material, anassociated change in refractive index occurs, which can be used tocreate various kinds of EO modulators, including phase, Mach-Zehndermodulators and directional coupler modulators. The change in refractiveindex is directly proportional to the applied electric field, so thatthis is sometimes called the linear EO effect, which only occurs inmaterials lacking a center of symmetry, as opposed to the much smallerquadratic EO effect which occurs in all materials. In most applications,the EO modulator is part of a fiber-optic communications system, inwhich case input and output fibers are aligned and attached to anoptical waveguide that has been created in the EO material by a varietyof methods. EO modulators are generally used when direct modulation ofthe laser in the communications system is not a viable option, whichoccurs when very high bandwidth (>10 GHz) or signal linearity arerequired. High bandwidth is needed in very high bit rate digitalcommunications systems (OC-768 systems running at 40 Gbps), while bothhigh bandwidth and high signal linearity are required for analogapplications such as phased array antennas, optical digital to analogconverters, and the like. The cw laser is typically a diode laser in thewavelength region from 800-1600 nm, predominantly near 1310 nm (O band)and near 1550 nm (C and L band).

The modulator is characterized by several critical parameters, the mostimportant of which are as follows:

-   -   Half-wave voltage, V_(π)—the voltage change needed to take the        modulator from its maximally transmitting state to its minimally        transmitting state; it is generally desired that this voltage be        as small as possible;    -   Insertion loss—the optical loss, in decibels, that is suffered        by light as it travels from the input to the output of the        modulator; it is always desired that the loss be as small as        possible;    -   Extinction—the ratio of the output power in the “on” state to        the output power in the “off” state; this ratio should be as        large as possible and is generally measured in decibels; and    -   3 dB bandwidth—the electrical driving signal frequency at which        the maximal optical output of the modulator, driven at the low        frequency half-wave voltage, has dropped by 3 dB or 50%.

For a Mach-Zehnder waveguide EO modulator, the half-wave voltage isrelated to the various other parameters of interest via:

${V_{\pi} = \frac{\lambda\; d}{n_{eff}^{3}r_{\max}\Gamma\; L}},$where λ is the optical wavelength, d is the physical separation distancebetween the drive electrodes in the direction along the applied field,n_(eff) is the effective refractive index for light polarized along thedirection in which the electric field is applied (generally a functionof both waveguide materials properties and dimensions), r_(max) is themaximum value of the EO coefficient (in units of picometers per volt)that can be achieved in the material, which dictates the preferreddirections for the applied field and the light polarization, Γ is thenormalized dimensionless overlap integral that measures the degree towhich the optical and electrical fields overlap, having a minimum valueof 0 and a maximum value of 1 and L is the length of the active regionof the modulator, defined as that region where an electric field isapplied to an EO material waveguide.

A hybrid EO polymer/sol-gel modulator was first described in Enami, et.al. Appl. Phys. Lett. 82, 490 (2003) and most recently reviewed inEnami, et. al., Nature Photonics 1, 180 (2007) and involves usingorganically modified sol-gels as the cladding and EO polymers as thewaveguide in the active region of the modulator and using just sol-gelsin the passive region of the modulator where coupling to the opticalfiber occurs. These sol-gels have an easily adjustable refractive index,which makes it possible to make waveguides with low coupling loss tooptical fiber. They also have been shown to allow for efficientin-device poling of the EO polymer as discussed in C. T. DeRose, et. al.Appl. Phys. Lett. 89, 131102 (2006) and U.S. Pat. No. 7,391,938.

As shown in FIG. 1, a hybrid EO polymer/sol-gel modulator 10 is formedon an insulating layer on a substrate (not shown). The modulatorincludes a bottom electrode 16, a sol-gel under cladding 18 having arefractive index n₁, a sol-gel core 20 having a refractive index n₂>n₁,and a sol-gel over cladding 22 having a refractive index n₁. The sol-gelcore 20 is confined below and on either side by under cladding 18. Inorder to provide a symmetric mode as the light propagates through thewaveguide and to provide better coupling to the input and output fibers,the cladding indices n₁ are preferably the same. However, some slightvariation (<<1%) may occur within a cladding or between cladding layersdue to slight variations in fabrication or design.

A vertical taper 23 in layer 22 exposes the surface of core 20 aboveelectrode 16. An EO polymer waveguide 24 having a refractive index n₃>n₂covers the exposed surface of the sol-gel core layer. In typicalembodiments, the refractive index n₃ is designed to be uniform over theactive region. Small variations from the uniform value on account offabrication and poling can occur without degrading performance. Anon-uniform index profile may be designed to provide the sameaccumulated phase change. A buffer layer 26 having a refractive indexn₄<n₂, typically <n₁ and preferably <n₁−0.1 covers the EO polymer layer.A top electrode 28 defines an active region 30 between itself and bottomelectrode 16 and passive regions 32 to either side. A voltage signalV_(sig) 34 is applied between the top and bottom electrodes to apply anelectric field along the poling direction of the EO polymer to changethe refractive index of the EO polymer waveguide and modulate theamplitude or phase of light 36 passing through the modulator. Modulator10 may be configured as a phase modulator 38 having one arm as shown inFIG. 2 a or as a Mach-Zehnder modulator 40 having a pair of arms asshown in FIG. 2 b and with the ability to produce on-chip amplitudemodulation. The Mach-Zehnder can be driven using single-arm, dual-driveor push-pull techniques well known to those in the art.

Light 36 can be input to and output from the modulator using a standardsingle-mode optical fiber (SMF-28 from Corning); the dimensions of theinput and output sol-gel waveguides (˜4 μm×4 μm) and the refractiveindex difference (˜1%) between the core and the cladding sol-gels can beoptimized so that coupling loss to SMF-28 is minimized. Light 36propagating in sol-gel core 20 proceeds and soon enters the region wherethe sol-gel over cladding 22 is physically tapered. As the lightpropagates further into this region it begins to “see” the EO polymerwaveguide 24 that has been deposited in the recessed region over andbetween the physical sol-gel tapers on the input and output sides of themodulator. Since the EO polymer has a significantly higher refractiveindex (˜1.6-1.7) than the sol-gel core (˜1.5), the light from thesol-gel core is gradually or “adiabatically” pulled up into the EOpolymer so that by the end of the taper as much light as possible hasbeen transferred to the EO polymer. The same mechanism (in reverse)results in the transfer of light from the EO polymer back to the sol-gelcore at the output. In the active region 30, the refractive index of thebuffer layer 26 is often chosen to be very low (˜1.3-1.4) which ensuresthat the optical waveguide mode in the EO polymer waveguide 24 drops offvery rapidly as it enters the buffer layer thereby minimizing lossesfrom the electrodes. In the case of a single-arm Mach-Zehnder modulator(FIG. 2 b), when a field is applied to the electrodes and light ispropagating in the modulator, light propagating in the left-hand arm ofthe Mach-Zehnder receives a phase shift relative to the lightpropagating the right-hand arm, so that the intensity at the output ofthe Mach-Zehnder changes.

In a representative embodiment sol-gel claddings 18 and 22 and core 20are comprised of 95/5 (n=1.487) and 85/15 (n=1.50) mixtures (mole %) ofmethacryloyloxy propyltrimethoxysilane (MAPTMS) andzirconium-IV-n-propoxide, where the MAPTMS provides both organic andinorganic character as well as photopatternability, while thezirconium-IV-n-propoxide is used as an index modifier. The undercladding 18 is deposited by spin coating and hard baked (˜8.5 μm), whilethe core 85/15 20 is also deposited by spin coating, but then issoft-baked and photopatterned, through a simple wet etching process,prior to hard baking (˜4 μm). Under cladding, also 18, is depositedafter the core is patterned to provide confinement on either side of thecore. This photopatterned sol-gel waveguide provides excellent couplingto standard SMF-28 single-mode fiber, with coupling losses in the0.5-1.0 dB per end face range routinely achieved. Propagation losses forthis standard sol-gel are 3-4 dB/cm. An adiabatic vertical transition toan EO polymer waveguide (n˜1.65) is accomplished through the use of agrey scale mask that creates a vertical taper in the next layer ofsol-gel (i.e. sol-gel over cladding 22). Note that this is not anevanescent field device, as the vertical adiabatic transitions result inmore than 70% of the optical field being in the EO polymer waveguide 24.The vertical tapers are also low loss, with less than 1 dB of radiationloss at each taper when the device is made to design. The EO polymerwaveguide 24 is typically quite thin (˜1 μm) to achieve single-modeoperation; the sol-gel over cladding 22 provides lateral confinement.The buffer layer 26 consists of CYTOP® (Asahi Glass), which has a verylow refractive index (n=1.33) at 1550 nm as well as extremely lowoptical absorption, thereby providing good optical isolation from thetop drive electrode 28 even for thin layers (˜1.5 μm).

SUMMARY OF THE INVENTION

The present invention provides hybrid EO polymer/sol-gel modulators withhigher electric field/optical field overlap factor Γ and smallerinter-electrode separation d to reduce the modulator's half-wave drivevoltage V_(π), reduce insertion loss and improve extinction. This isaccomplished with modulator designs in which the sol-gel core waveguidedoes not lie below the EO polymer waveguide in the active region.

In an embodiment, a hybrid electro-optic (EO) polymer/sol-gel modulatorcomprises a sol-gel core waveguide in first and second passive regionsfor coupling light into and out of the modulator. An EO polymerwaveguide in an active region between said first and second passiveregions provides for transmission of light from said first passiveregion through said active region to said second passive region. Thesol-gel waveguide is not established below the EO polymer waveguide inthe active region but instead may provide a cladding for the EO polymerwaveguide in the active region. First and second electrodes on oppositesides of the EO polymer waveguide in the active region are configured toreceive a voltage signal to apply an electric field along a polingdirection to change the refractive index of the EO polymer waveguide inthe active region.

In a first index-tapered, core-tapered embodiment, the sol-gel overcladding has been eliminated and the vertical taper formed in thesol-gel core. Placing the EO polymer waveguide in the same plane as thesol-gel core eliminates the potential for deleterious coupling betweenthe sol-gel core and the EO polymer waveguide. The EO polymer waveguideis physically extended to cover the vertical taper in the passiveregions on either side of the active region. The index itself is taperedfrom a high uniform value in the active region to a lower value toapproximately match the sol-gel core at the top of the taper. Bytapering the EO polymer waveguide index the inter-electrode separation dis reduced and the electric field/optical field overlap factor Γ isincreased, both having the beneficial effect of reducing the half-wavedrive voltage. The formation of the vertical taper in the sol-gel coreand the index taper of the EO polymer waveguide are separable features.

In a second phantom-core embodiment, the sol-gel core lies in a planebelow the plane of the EO polymer waveguide. The sol-gel core ispatterned so that the core waveguide stops at or near the end of thevertical taper in the sol-gel over cladding. The gap that is created isfilled with a sol-gel or other material with a refractive index lessthan or equal to that of the sol-gel under cladding. Since this regionis no longer a waveguide, no light is coupled from the EO polymerwaveguide to the core thereby increasing Γ and reducing coupling to thebottom electrode. In fact, the bottom electrode may be placed on top ofthe sol-gel under cladding for reduced inter-electrode separation d. Thelack of background light from the sol-gel core leads to significantlyimproved modulation efficiency as well.

In a third transverse-tapered embodiment, a sol-gel side-cladding liesin the same plane as the sol-gel core. The side-cladding has atransverse physical taper on one (phase modulator) or both (Mach-Zehndermodulator) sides of the sol-gel core that defines a gap on one or bothsides of the core. The gap increases gradually from the core from bothends until reaching a fixed gap that is maintained over the activeregion of the modulator. The gap(s) between the sol-gel core and sol-gelside-cladding are filled with EO polymer to form an EO polymerwaveguide. A buffer layer lies over the EO polymer. A top electrode andpatterned bottom electrode define the active region there between. Thistransverse-taper design improves confinement in the EO polymer waveguideso that the inter-electrode spacing can be reduced and Γ increased. Thetransverse physical taper can be formed using standard lithographictechniques. The performance can be improved by tapering the refractiveindex of the EO polymer waveguide to gradually increase from a valueslightly above that of the sol-gel core to its ordinary high value inthe active region of the EO modulator.

A fourth strip-loaded embodiment comprises a sol-gel under claddinghaving a refractive index n₁, a sol-gel core having a refractive indexn₂>n₁ and a sol-gel side cladding having a refractive index n₃≈n₁<n₂ inthe same plane as the sol-gel core. An EO polymer layer is provided witha graded refractive index. The EO polymer has an index n₄<n₂ in a firstpassive region that provides a top cladding for the sol-gel core todefine a first sol-gel waveguide in the first passive region. The indexis graded in a first transition region from n₄ up to n₅>n₂ in an activeregion so that the sol-gel core provides a bottom cladding for the EOpolymer layer to define an EO polymer waveguide in the active region,The index is graded in a second transition region from n₅ down to n₆<n₂in a second passive region that provides a top cladding for the sol-gelcore to define a second sol-gel waveguide in the second passive region.A sol-gel buffer layer lies above the EO polymer layer and has arefractive index n₇<n₅ in the active region. First and second electrodeson opposite sides of the EO polymer waveguide in the active region areconfigured to receive a voltage signal to apply an electric field alongthe poling direction to change the refractive index of the EO polymerwaveguide in the active region. The use of a uniform thickness EOpolymer layer eliminates optical scattering caused by sidewall roughnessdue to etching, thereby leading to lower optical propagation loss in theactive section. The sol-gel core also acts as a virtual side-cladding toconfine light laterally in the overlying EO polymer layer therebycreating a well defined EO polymer waveguide in the active region forefficient coupling back into the sol-gel waveguide for coupling tooptical fiber.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, as described above, is a section view of they hybrid EOpolymer/sol-gel modulator;

FIGS. 2 a and 2 b, as described above, are perspective views of a hybridEO polymer/sol-gel modulator configured as a phase modulator and aMach-Zehnder modulator, respectively;

FIGS. 3 a and 3 b are section and end views of an embodiment of a hybridtapered-core tapered-index Mach-Zehnder modulator in accordance with thepresent invention;

FIG. 4 is a plot of the tapered-index of the EO polymer waveguide;

FIGS. 5 a and 5 b are diagrams illustrating mode propagation through theknown and hybrid tapered-index, tapered-core Mach-Zehnder modulators,respectively;

FIG. 6 is a section view of another embodiment of a hybrid tapered-indexmodulator;

FIG. 7 is a section view of another embodiment of a hybrid tapered-coremodulator;

FIGS. 8 a and 8 b are section views of two embodiments of a hybridphantom-core Mach-Zehnder modulator in accordance with the presentinvention;

FIGS. 9 a and 9 b are diagrams illustrating mode coupling through theknown and phantom-core modulators, respectively;

FIGS. 10 a through 10 d are plan and section views in the passive,transition and active regions of another embodiment of a hybridtransversely-tapered modulator;

FIGS. 11 a through 11 c are plots of the optical power distribution forthe mode propagation in the transversely-tapered modulator;

FIGS. 12 a through 12 e and 13 a through 13 e are a section in theaction region and plan views of a sequence for fabricating thetransversely-tapered modulator;

FIGS. 14 a through 14 c are a side view, a section view B-B through thepassive region and a section view C-C through the active region,respectively, of a strip-loaded EO polymer/sol-gel modulator;

FIG. 15 is a plot of the EO polymer layer's graded refractive indexincluding an inset showing the grading of the index in a transitionregion from a uniform low value in the passive region to a uniform highvalue in the active region; and

FIG. 16 is a side section view of an exemplary strip-loaded EOpolymer/sol-gel modulator.

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes hybrid EO polymer/sol-gel modulatorswith higher electric field/optical field overlap factor Γ and smallerinter-electrode separation d to reduce the modulator's half-wave voltageV_(π), reduce insertion loss and improve extinction. Applications forthese new designs include but are not limited to high speed analogmodulators for cable TV, phased array radars, photonic analog-to-digitalconverts, RF photonics, fiber optic gyroscopes and true time delay linesas well as digital modulators for optical communications networksranging from long haul dense wavelength division multiplexing (DWDM)networks, to metropolitan area DWDM, to passive optical networks (PON)for optical access.

The conventional design of the hybrid EO polymer/sol-gel modulator asillustrated in FIG. 1 suffers from several limitations owing principallyto the fact that the design employs a sol-gel core waveguide that liesdirectly below the active EO polymer waveguide—this has the effect ofallowing light to couple from the EO polymer waveguide to the sol-gelcore in the active region of the device resulting in the followingperformance impairments:

-   -   Since the light in the sol-gel core is optically close to the        bottom electrode, the bottom cladding must be made thick enough        to assure that the light does not reach the electrode thereby        causing high insertion loss; this extra thickness results in        increased required electrode separation d, which increases the        half-wave voltage; a typical separation required is 15 μm;    -   The residual light that is in the sol-gel core does not get        modulated, leading to a lower extinction ratio—a typical        extinction ratio achieved is 10 dB, whereas most commercial        applications require 20 dB or higher; and    -   The coupling of the light into the sol-gel core and the        limitation of the tapering technique used in the prior art lead        to a moderate value of Γ, typically ˜0.7

The hybrid electro-optic (EO) polymer/sol-gel modulator comprises asol-gel core waveguide in first and second passive regions for couplinglight into and out of the modulator. An EO polymer waveguide in anactive region between said first and second passive regions provides fortransmission of light from said first passive region through said activeregion to said second passive region. The sol-gel waveguide is notestablished below the EO polymer waveguide in the active region butinstead may provide a cladding for the EO polymer waveguide in theactive region. First and second electrodes on opposite sides of the EOpolymer waveguide in the active region are configured to receive avoltage signal to apply an electric field along a poling direction tochange the refractive index of the EO polymer waveguide in the activeregion.

The present invention provides four different designs that do not employa sol-gel core waveguide that lies below the EO polymer waveguide in theactive region. All four of the new designs result in improvements in d,Γ, d/Γ and V_(π). At a minimum, d can be reduced to 10 μm from thebaseline art value of 15 μm, resulting in at least a 33% reduction inV_(π). Γ can be improved to at least 0.8 from its baseline art value of0.7, representing at least a 14% reduction in V_(π). Therefore, theratio d/Γ is, at a minimum, reduced to 12.5 μm, compared to the priorart value of 21.5 μm. Thus, the V_(π) is at a minimum reduced by 42%from its prior art value.

More particularly, in one or more of the different designs the electrodeseparation d is at a minimum reduced by 33% from 15 μm to 10 μm,preferably reduced to 8 μm (47% reduction) and at a maximum reduced to 6μm (60%). The overlap integral Γ is at a minimum increased to 0.8 fromthe prior art value of 0.7 (12.5%), typically increased to 0.85 (21%),and at a maximum increased to 0.95 (36%). The ratio d/Γ, which appearsin the expression for the half-wave voltage, is, at a minimum, reducedto 12.5 μm, a 42% reduction from its prior art value of 21.5, typicallyreduced to 9.4 μm (56%), and at a maximum reduced to 6.3 μm (70.7%reduction). Although the half-wave voltage V_(π) is dependent on otherparameters, the improvements in d and F should reduce V_(π) to less than1 V for many modulator designs.

Index-Tapered EO Polymer, Vertically-Tapered Sol-Gel Core Modulator

An index-tapered, core-tapered modulator eliminates the sol-gel overcladding and forms the vertical taper in the sol-gel core. Placing theEO polymer waveguide in the same plane as the sol-gel core eliminatesthe potential for coupling between the sol-gel core and the EO polymerwaveguide. The EO polymer waveguide is physically extended to cover thevertical taper in the passive regions on either side of the activeregion. The index is tapered from the high uniform value in the activeregion to a lower value to approximately match the sol-gel core at thetop of the taper in the passive regions. By tapering the EO polymerwaveguide index the inter-electrode separation d is reduced and theoverlap Γ between the optical and electrical fields is increased, bothhaving the beneficial effect of reducing the drive voltage V_(π). Theformation of the vertical taper in the sol-gel core and the index taperof the EO polymer waveguide are separable features.

As shown in FIGS. 3 a and 3 b, an embodiment of an index-tapered,core-tapered modulator 50 is formed on an insulating layer on asubstrate (not shown). The modulator includes a bottom electrode 56, asol-gel under cladding 58 having a refractive index n₁ and a sol-gelcore 60 having a refractive index n₂>n₁. Sol-gel under cladding 58provides confinement below and on either side of sol-gel core 60. Avertical taper 62 in core 60 exposes the surface of under cladding 58above electrode 56. An EO polymer waveguide 64 covers the vertical taper62 in the passive regions and the exposed surface of under cladding 58in the active region of the modulator. The EO polymer waveguide exhibitsa refractive index 65 having a high value n₃>n₂ in the active region 70that tapers in a transition region 71 to a lower value n₄≈n₂ (e.g.within +/−2%) at the top of the physical taper in the passive region 72as shown in FIG. 4. Note, the ‘transition’ regions 71 for the indextaper are part of the passive regions 72. In typical embodiments, therefractive index n₃ is designed to be uniform over the active region.Small variations from the uniform value on account of fabrication andpoling can occur without degrading performance. A non-uniform indexprofile may be designed to provide the same accumulated phase change.

A buffer layer 66 having a refractive index n₅<n₂, typically <n₁ andpreferably <n₁−0.1 covers the EO polymer waveguide. The index of thebuffer layer is preferably considerably less than core index n₂. The EOpolymer waveguide 64 is laterally confined by sol-gel under cladding 58and vertically confined by buffer layer 66 and sol-gel under cladding 58as shown in FIG. 3 b. A top drive electrode 68 defines the active region70 between itself and bottom electrode 56 and passive regions 72 toeither side. An analog or digital voltage signal V_(sig) 74 is appliedbetween the top and bottom electrodes to change the refractive index ofthe EO polymer waveguide to modulate the amplitude or phase of light 76passing through the modulator. Modulator 50 is configured as aMach-Zehnder modulator but may be configured as a phase modulator.

In a representative embodiment of modulator 50 the sol-gel undercladding 58 and core 60 are comprised of 95/5 (n₁=1.487@1550 nm) and85/15 (n₂=1.50@1550 nm) mixtures (mole %) of methacryloyloxypropyltrimethoxysilane (MAPTMS) and zirconium-IV-n-propoxide. The undercladding 58 is deposited by spin coating and hard baked (˜8.5 μm), whilethe core 85/15 layer is also deposited by spin coating, but then issoft-baked and photopatterned to form core 60, through a simple wetetching process, prior to hard baking (˜4 μm). Cladding, also 58, isdeposited after the core and patterned to provide confinement on eitherside of the core. The preferred sol-gel material will have a highconductivity at poling temperatures to provide for efficient poling ofthe EO polymer, refractive indices n₁ and n₂ such that their difference(n₂−n₁) is approximately equal to that of standard single-mode opticalfiber (˜0.01), and low optical loss at 1550 nm and 1310 nm, preferablyless than 1.0 dB/cm at both wavelengths. The EO polymer should exhibit ahigh EO coefficient r₃₃>30 pm/V and preferably >60 pm/V. Unlike theknown hybrid modulator design, the sol-gel core material lies outsidethe region of polymer poling, hence the sol-gel core does not have to beable to provide high poling efficiency. This removes a constraint,possibly allowing for a larger class of sol-gel materials to be used ineach of the proposed designs. Furthermore, the proposed designs reduceinter-electrode spacing. To achieve the same performance, an EO polymerwith a lower r₃₃ can be used with thinner devices. These materials mayprovide better stability and lower optical loss. A 6 μm thick thermallygrown oxide SiO₂ layer beneath the sol-gel under cladding 58 is used topreserve optical transparency at 1550 nm wavelength and prevent couplingto the silicon substrate in the passive regions.

The EO polymer layer 64 is spin coated on the entire device and istypically quite thin (˜1 μm) to achieve single-mode operation. Thesol-gel core layer with a thickness of 4 μm has a refractive indexdifference of 0.86% from the sol-gel under cladding (5 μm) andside-cladding to increase the efficiency of the optical coupling tostandard single mode fiber (SMF-28™), and is vertically tapered 62 toproduce an adiabatic transition between the sol-gel core 60 and the EOpolymer waveguide 64. To improve coupling, vertical taper 62 ispreferably a long shallow taper e.g. 3.5 μm change in height over a 1millimeter length or 0.0035 radians measured from the surface. An angleof taper less than 0.01 radians is suitable; less than 0.007 radians istypical and less than 0.005 radians being preferred. In an embodiment,the taper is at least 0.4 mm in length with an angle less than 0.01radians.

An electrode is placed directly on the top of the EO polymer waveguidefor poling and then removed. After poling, the modulator is irradiatedwith UV radiation (9 mW/cm²) through a gray scale mask for 18 h tofabricate photobleached index tapers in the EO polymer waveguide 64 inthe interface regions. The photobleached portion in the passive regionswill become transparent, and the interface between the active andpassive regions will evidence a gradual change from the original greento the colorless host polymer. The index of the photobleached EO polymersingle film coated on SiO₂ was measured using the prism coupling method,and showed gradual index change from n₃=1.65 to n₄=1.50 after UVradiation. In a typical embodiment, n₃ is substantially uniform in theactive region for a given device. The index value typically lies in arange from approximately 1.53 to approximately 1.80.

The buffer layer 66 is coated and the operating top electrode 68 isdeposited. The electric field is applied between the electrodes alongthe poling direction to maximize the EO effect. The inter-electrodespacing is 15 μm or less. The buffer layer 66 consists of 2.0 μm-thickUV cured acrylate top buffer layer (n₅=1.49). The buffer layer can beany material with reasonably low optical loss at telecommunicationswavelengths (<3 dB/cm) with an index that is less than n₂ and typicallyconsiderably less than the core index. The buffer layer principallyserves to isolate light in the active EO polymer waveguide from the topelectrode. In the event that the poling is performed directly on the EOpolymer waveguide (and the electrode removed prior to the application ofthe buffer layer), the buffer layer must be a low temperature curing ordrying material, so as to avoid depoling the EO polymer. If the polingis done after buffer layer deposition, the buffer layer should haveconductivity higher than that of the EO polymer at the polingtemperature in order to achieve efficient poling.

In the described embodiment, the EO polymer waveguide 64 is poled in thevertical direction. The electrodes 56 and 68 are placed to create anelectric field along the poling direction for maximum effect. In analternative embodiment, the EO polymer is poled in the horizontaldirection and the electrodes are formed co-planar. This approach makespoling the EO polymer more difficult and may constrain the availableaddressing techniques but simplifies the microwave engineering of themodulator. Each of the index-tapered/core-tapered, phantom-core andtransversely-tapered may be configured with either the vertical orco-planar electrodes, the general requirement being that the electrodescreate an electric field along the poling direction of the EO polymer.

Compared to the known hybrid modulator design 10 shown in FIG. 1,modulator 50 exhibits improved mode confinement. The adiabatictransition is accomplished using a combination of a 0.5 mm-long sol-gelvertical taper 62 and a refractive index taper in the EO polymerwaveguide 64, which reduce the transition loss and prevents mode beatingbetween the sol-gel core and the EO polymer waveguide. Improved modeconfinement in the EO polymer waveguide in the active region alsoreduces waveguiding loss due to optical absorption from under and overelectrodes. As shown in FIG. 5 a for the known modulator 10 and FIG. 5 bfor index-tapered core-tapered modulator 50, the confinement of light 36and 76, respectively, propagating through the modulators is markedlydifferent. In the figures, the optical power density is highest wherethe dot density is lowest, and falls off rapidly with increasing dotdensity. Light 36 spreads out through the modulator, with much of thelight in the cladding. Light 76 is much better confined to the EOpolymer waveguide. Consequently the inter-electrode separation distanced can be reduced and the overlap factor Γ is increased.

In an alternate embodiment of the hybrid modulator 80 as shown in FIG.6, a vertical taper 82 is formed in a sol-gel over cladding 83 havingindex n₁ above a sol-gel core 84. The over and under cladding indices n₁are preferably the same. However, some slight variation (<<1%) may occurwithin a cladding or between claddings due to slight variations infabrication or possibly design. For simplicity of description bothcladdings are designated as having index n₁ but slight variations intheir values are well understood by those skilled in the art. An EOpolymer waveguide 86 having a refractive index taper is formed over thevertical taper 82 and an exposed portion of the sol-gel core. The EOpolymer waveguide index tapers to a value approximately equal to that ofthe over cladding. The other aspects of the modulator are the same.

In an alternate embodiment of the hybrid modulator 90 as shown in FIG.7, a vertical taper 92 is formed in a sol-gel core 94. An EO polymerwaveguide 96 having a uniform refractive index is formed inside thetaper over an exposed portion of the sol-gel under cladding 98. Theother aspects of the modulator are the same.

Phantom-Core Modulator

A phantom-core modulator includes a patterned sol-gel core that stops ator near the end of the vertical taper in the sol-gel over cladding. Thegap that is created in the core is filled with a sol-gel or othermaterial with a refractive index less than or equal to that of thesol-gel under cladding. Since this region is no longer a waveguide, nolight is coupled from the EO polymer waveguide to the core. If the indexdifference between the phantom core and the EO polymer waveguide islarge enough, the bottom electrode may be placed on top of the sol-gelunder cladding for reduced inter-electrode separation. The lack ofbackground light from the sol-gel core layer leads to significantlyimproved modulation efficiency.

As shown in FIG. 8 a, a hybrid EO polymer/sol-gel modulator 100 isformed on an insulating layer of a substrate (not shown). The modulatorincludes a bottom electrode 106, a sol-gel under cladding 108 having arefractive index n₁, a sol-gel core 110 having a refractive index n₂>n₁,and a sol-gel over cladding 112 having a refractive index n₁. The core110 is confined below and to either side by under cladding 108. The overand under cladding indices n₁ are preferably the same. However, someslight variation (<<1%) may occur within a cladding or between claddingsdue to slight variations in fabrication or possibly design. For purposesof description both claddings are designated as having index n₁ butslight variations in their values are well understood by those skilledin the art.

The sol-gel core is patterned to form a gap 114 that exposes the surfaceof the under cladding 108 so that the core stops at or near the end of avertical taper 116 in over cladding 112. Gap 114 is filled with asol-gel or other material 118 with a refractive index n₃<=n₁, preferablyat least 0.10 less than and most preferably at least 0.15 less than. Thematerial should exhibit low optical loss and reasonably highconductivity at typical poling temperatures. The taper exposes thesurface of material 118 above electrode 106. An EO polymer waveguide 120having a refractive index n₄>n₂ covers the exposed surface of material118. In typical embodiments, the refractive index n₃ is designed to beuniform over the active region. Small variations from the uniform valueon account of fabrication and poling can occur without degradingperformance. A non-uniform index profile may be designed to provide thesame accumulated phase change.

A buffer layer 122 having a refractive index n₅<n₂, typically <n₁ andpreferably <n₁−0.1 covers the EO polymer waveguide. A top driveelectrode 124 defines an active region between itself and bottomelectrode 106 and passive regions to either side. An analog or digitalvoltage signal is applied between the top and bottom electrodes tochange the refractive index of the EO polymer waveguide to modulate theamplitude or phase of light 126 passing through the modulator. Modulator100 may be configured as a phase modulator or as a Mach-Zehndermodulator. If the index of material 118 is not at least 0.10 less thanthe index of the under cladding 108, bottom electrode 106 should beplaced beneath the under cladding 108 to avoid mode coupling to thebottom electrode as shown in FIG. 8 b. Alternatively, the electrodes 106and 124 can be co-planar and the EO polymer 120 poled along a horizontaldirection. In another embodiment, the EO polymer can be index tapered inthe physical taper region and over the passive sol-gel upper cladding soas to have the refractive index approximately match that of the sol-gelin the passive region, while gradually increasing in the physical taperregion.

As shown in FIG. 9 a, the known hybrid modulator design shown in FIG. 1permits directional coupling of light 36 between the EO polymerwaveguide 24 and the sol-gel core 20. Light remaining in the sol-gelcore leads to increased insertion loss and low contrast. As shown inFIG. 9 b, the phantom core design eliminates this possibility; no lightis coupled between EO polymer waveguide 120 and material 118 andprovides for reduced inter-electrode distance.

Transversely-Tapered Modulator

The vertical coupling approach exemplified by the known hybrid modulatorof FIG. 1 and the improved index-tapered/core-tapered and phantom-coredesigns demands excellent control over film thicknesses in all regionsof the device, which can be both difficult and costly. A horizontally(transversely) coupled device in which the sol-gel core waveguide doesnot lie below the active EO polymer waveguide has fewer issues withrespect to dimensional control, relying primarily upon lithographicallydefined features whose resolution is well beyond the requirements of themodulator.

A transversely-tapered modulator includes a sol-gel side-cladding thatlies in the same plane as the sol-gel core. The side-cladding has atransverse physical taper on one (phase modulator embodiment) or both(Mach-Zehnder modulator embodiment) sides of the sol-gel core thatdefines a transverse gap on one or both sides of the core. The gapincreases gradually from the core from both ends until reaching a fixedgap that is maintained over the active region of the modulator. Thegap(s) between the sol-gel core and sol-gel side-cladding are filledwith EO polymer to form an EO polymer waveguide(s). A buffer layer liesover the EO polymer. A top electrode and patterned bottom electrodedefine the active region there between. This transverse-taper designimproves confinement in the EO polymer waveguide so that theinter-electrode spacing can be reduced. The transverse physical tapercan be formed using standard lithographic techniques. The performancecan be improved by tapering the refractive index of the EO polymerwaveguide to gradually increase from a value slightly above that of thesol-gel core to its ordinary high value in the active region of the EOmodulator.

As shown in FIGS. 10 a through 10 d a transversely-tapered modulator 200is formed on an insulating layer of a substrate (not shown). Themodulator includes a patterned bottom electrode 206, a sol-gel undercladding 208 having a refractive index n₁ and a sol-gel core 210 havinga refractive index n₂>n₁. A sol-gel side-cladding 212 having arefractive index n₁ lies in the same plane as sol-gel core 210. Theunder and side cladding indices n₁ are preferably the same. However,some slight variation (<<1%) may occur within a cladding or betweencladdings due to slight variations in fabrication or possibly design.For purposes of description both claddings are designated as havingindex n₁ but slight variations in their values are well understood bythose skilled in the art.

The side-cladding has a transverse physical taper 214 on one (phasemodulator embodiment) or both (Mach-Zehnder modulator embodiment) sidesof the sol-gel core that defines a transverse gap on one or both sidesof the core. The gap increases gradually from the core from both endsover lengths L_(taper1) and L_(taper2), respectively, until reaching afixed gap that is maintained over the active region L_(active) of themodulator. The gap(s) between the sol-gel core and sol-gel side-claddingare filled with EO polymer having refractive index n₃>n₂ to form an EOpolymer waveguide(s) 216. In typical embodiments, the refractive indexn₃ is designed to be uniform over the active region. Small variationsfrom the uniform value on account of fabrication and poling can occurwithout degrading performance. A non-uniform index profile may bedesigned to provide the same accumulated phase change. Performance canbe improved by tapering the refractive index from value n₃ overL_(active) over lengths L_(taper1) and L_(taper2) in the passive regionsto a value n₅ to approximately match index n₂ of the sol-gel coresimilar to what is depicted in FIG. 4.

A buffer layer 218 having a refractive index n₄<n₂, typically <n₁ andpreferably <n₁−0.1 lies over the EO polymer. A top electrode 220 andpatterned bottom electrode 206 define the active region there between.As shown in this embodiment, a very thin layer of EO polymer 221 liesbetween core 210 and buffer layer 218. This is a residual of thestandard spin-on processing of the EO polymer and is opticallyinsignificant. This layer is not poled or modulated by the electricfields created by the electrodes to either side. The residual layercould be removed using dry-etch techniques such as reactive ion etching(RIE), but is unnecessary.

The input and output regions of the transverse-tapered modulator aresimilar to those of the vertically-tapered modulator. Sol-gel core 210and cladding 208 create a single-mode optical waveguide with excellentcoupling to optical fiber. Light 222 coupled into the modulator from aninput SMF is initially confined to travel in sol-gel core 210 andgradually couples transversely into EO polymer waveguide(s) 216 in whichthe light is modulated in accordance with changes in the refractiveindex of the EO polymer induced by the applied electric field. Themodulated light 222 is coupled back into core 210 and coupled to theoutput SMF. The Mach-Zehnder waveguide modulator does not requireY-branches, light 222 traveling in the core splits in two with halftraveling down each EO polymer waveguide 216. The transverse designimproves confinement of light in the EO polymer waveguide in the activeregion, which means the inter-electrode separation can be reducedwithout deleterious optical coupling to either electrode. This in turnmeans that the V_(π) can be reduced.

Optical power distribution for the mode propagation calculated using the3D beam propagation method (BPM) is illustrated in FIGS. 11 a-11 c withthe X, Y, and Z axes correspond to transverse, vertical, and propagationdirections. The top view shown in FIG. 11 a illustrates the integratedmode power for the total waveguide, the EO polymer core, and the sol-gelcore layers. Light is confined to the sol-gel core 210 in the passiveregions and the EO polymer waveguide 216 in the active regions. The sideview shown in FIG. 11 b illustrates the center region of the sol-gelstraight core 210, demonstrating that no light is in the sol-gel core inthe active region. The side view shown in FIG. 11 c illustrates thecenter of an EO polymer MZ waveguide arm 216 (both are identical). Thenegative regions on the Y axis in FIG. 11 b refer to positions in theunder cladding 208.

Strong mode confinement (>90%) was calculated for the EO polymer coresignificantly exceeding that of previous hybrid modulators. Thetransverse transition loss was calculated to be <0.2 dB. This reduces dand increases Γ.

As shown in FIGS. 12 a-12 e and 13 a-13 e, a transverse-taper modulator250 can be fabricated using standard photolithographic masking and highprecision lithography. This is simpler, less expensive and more precisethan the grey-scale masking techniques required to form the verticaltaper. A fabrication sequence for a particular embodiment of themodulator is now described.

A multilayer bottom electrode 252 of Cr(10 nm)/Au(100 nm)/Cr(10 nm) iselectron beam evaporated onto an Si<100> substrate 254 with a 6-μm-thickthermally grown SiO₂ layer (not shown). A lift-off technique is used topattern the bottom electrode for dual-drive modulation. An undercladding 258 3 to 3.7 μm thick of methacryloyloxy propyltrimethoxysilane(MAPTMS) doped with an index modifier, zirconium (IV)-n-propoxide (ZrPO)having a mole ratio of MAPTMS to ZrPO of 95/5% is spin-coated over thesubstrate. UV radiation (λ_(peak)=365 nm, 9 mW/cm²) accelerates theformation of the silica network in the sol-gel layers via thephotoinitiator Irgacure 184 (CIBA). The cladding layers are UV-radiatedthrough a photolithographic mask and wet-etched in isopropanol forphotopatterning.

After coating and baking (150° C./1 h) of the sol-gel under cladding 258a 3.5-μm-thick core layer was coated and wet-etched to give an 8-μm-widestraight channel core 260. The core layer is formed from the samematerial as the cladding but with a mole ratio of 85/15%.

A 7-μm-thick side cladding layer is coated and wet-etched to provide aside-cladding 262 and trenches 264 for the tapers and trenches 266 the4-μm-wide MZ EO polymer channel waveguide. To maximize the couplingefficiency of the hybrid waveguide with SMF-28™, the top claddingcovering the core 260 in the passive regions was coated and wet-etchedto deposit the top cladding only in the passive region, and finallybaked at 150° C. for 1 h.

A 2.8-μm-thick-EO polymer layer 268 is deposited by spin coating from acyclopentanone solution to fill the trenches 264 and 266 adjacent to thesol-gel core 260 on the sol-gel under cladding 258. The guest-host EOpolymer (25 wt % chromophore loaded AJLS102 in amorphous polycarbonate)was used as it readily photobleaches, has relatively low optical loss,and a good EO coefficient (˜50 μm/V). After baking at 85° C. in a vacuumoven overnight, the EO polymer was photobleached using UV radiationthrough a gray-scale mask for 18 h. The EO polymer was partiallybleached with a gradual index change in 0.5-mm-long transition regions,and totally bleached (n=1.53) in the other passive regions. After the EOpolymer was contact-poled (poling voltage of 300-400V at 150° C.)between the bottom electrode and a poling electrode directly depositedonto the EO polymer, the poling electrode was removed using a solutionof iodine and potassium iodide for subsequent coating of a low index(˜1.33@1550 nm) top buffer layer. A 1.8-μm-thick CYTOP (Asahi Glass) topbuffer layer 270 is formed over the EO polymer waveguide 268 and gold issputtered to form a top electrode 272. The refractive indices at 1550 nmfor the sol-gel core, the cladding, the EO polymer, and buffer layerwere 1.500, 1.487, 1.632, and 1.328, respectively.

Strip-Loaded Modulator

The vertical coupling approach exemplified by the known hybrid modulatorof FIG. 1 and the improved index-tapered/core-tapered, phantom-coredesigns and transverse tapered modulators all achieve excellent drivevoltage performance (as low as 0.65V has been achieved) and reasonablygood insertion loss (about 10 dB). For many applications, especially inRF photonics, very low insertion loss is required on the order of 5.5 dBor less. To achieve this level of insertion loss a further modificationto the basic design of FIG. 1 was developed, namely a strip loadedmodulator.

A strip-loaded modulator includes a sol-gel side-cladding that lies inthe same plane as the sol-gel core. The side-cladding index is chosen soas to achieve an optical mode shape at the input to the device thatmatches well to the mode field diameter of high numerical apertureoptical fiber (NA˜0.2). A uniform EO polymer layer is deposited on topof the sol-gel core layer. In the passive section of the device, the EOpolymer layer may be photobleached so that the refractive index of thebleached EO polymer is below that of the sol-gel core, in which case thebleached EO polymer acts as a top cladding for the sol-gel core A topelectrode and patterned bottom electrode define the active region therebetween. The refractive index of the EO polymer layer is made togradually increase from the passive region, where it is a top claddingfor the sol-gel core, through a transition region to the active region,where the sol-gel now acts as an under cladding for the high index EOpolymer, which acts to strip load the waveguide stack. Application of avoltage across the active region modulates the refractive index of theEO polymer, which in turn produces amplitude changes in the light beam.At the output side of the modulator, the EO polymer index is reducedagain to its value at the input, so that light will be guided once againin the sol-gel core and coupled out to optical fiber.

The use of a uniform thickness EO polymer layer eliminates opticalscattering caused by sidewall roughness due to etching, thereby leadingto lower optical propagation loss in the active section. This is theprimary origin of its significantly reduced insertion loss, in additionto excellent mode matching to optical fiber and low loss verticaladiabatic transitions. The sol-gel core and sol-gel buffer layer in theactive region act as an under and top cladding that confine the lightvertically in the EO polymer layer. The sol-gel core also acts as avirtual side-cladding to confine light laterally in the overlying EOpolymer layer thereby creating a well defined EO polymer waveguide inthe active region.

As shown in FIGS. 14 a (side), 14 b (passive) and 14 c (active) astrip-loaded modulator 300 is formed on an insulating layer of asubstrate (not shown). The modulator includes a sol-gel under cladding302 having a refractive index n₁ and a sol-gel core 304 having arefractive index n₂>n₁. A sol-gel side-cladding 306 having a refractiveindex n₃ (with n₃<n₂ and n₃≈n₁) lies in the same plane as sol-gel core310. The under and side cladding indices n₁ and n₃ are suitably thesame. However, some slight variation (<<1%) may occur within a claddingor between claddings due to slight variations in fabrication or possiblydesign. The index of the sol-gel core is uniform across the passive andactive regions.

An EO polymer layer 312 lies over the sol-gel core 304 and side cladding306. The EO polymer layer is of uniform thickness and not etched withinthe footprint of the transition or active regions. As best shown in FIG.15, the EO polymer layer 312 has a graded refractive index 313 along thelength of the modulator 300. The EO polymer layer has an index n₄<n₂ ina first passive region 314 that provides a top cladding for the sol-gelcore to define a first sol-gel waveguide in said first passive region.The index is graded in a first transition region 316 from n₄ up to n₅>n₂in an active region 318 so that the sol-gel core provides a bottomcladding for the EO polymer layer to define an EO polymer waveguide inthe active region. The index is graded in a second transition region 320from n₅ down to n₆<n₂ in a second passive region 322 that provides a topcladding for the sol-gel core to define a second sol-gel waveguide inthe second passive region. n₄ may ≈n₆. The difference in refractiveindex in the EO polymer layer between the passive and active regions maybe at least 0.015. In the passive region, the refractive index of thesol-gel core may be at least 0.0075 greater than the index of the EOpolymer layer. In the active region, the refractive index of the EOpolymer layer may be at least 0.0075 greater than the index of theunderlying sol-gel. The graded index may be formed by photobleaching theEO material in the passive and transition regions to reduce therefractive index. In the active region of the device the EO polymer maybe unbleached. Alternately, direct write electron beam lithography maybe used to destroy the chromophore in the EO polymer to form the gradedindex. The EO polymer layer is generally poled along a directionperpendicular to the substrate, but can also be poled transversely (inthe plane of the substrate).

A top sol-gel buffer layer 324 lies above the EO polymer 312. The bufferlayer has an index n₇ where n₇<n₅ in the active region to provide anupper cladding for the EO waveguide. n₇ may ≈n₁.

A top electrode 326 and patterned bottom electrode 328 define the activeregion 318 there between in which the EO polymer waveguide is defined.The electrodes are configured to receive a voltage signal to apply anelectric field along the poling direction to change the refractive indexof the EO polymer waveguide in the active region.

The input and output regions of the strip-loaded modulator arequalitatively similar to those of the other embodiments. Sol-gel core304 and claddings 302, 306, 324 and the bleached EO polymer layer 312create a single-mode optical waveguide with excellent coupling tooptical fiber. Light 330 coupled into the modulator from an input SMF isinitially confined to travel in the sol-gel waveguide defined in sol-gelcore 304 and traverses transitions region 316 to couple into the EOpolymer waveguide defined in EO polymer layer 312 in which the light 330is modulated in accordance with changes in the refractive index of theEO polymer induced by the applied electric field. Single-mode as usedherein means that the waveguide is single-mode both in the plane andperpendicular to the plane of the substrate. FIG. 15 shows a typicalrefractive index profile 313 achieved in the EO polymer in order toefficiently transfer the light from the sol-gel core (passive region314) to the EO polymer (active region 318).

As shown in FIG. 14 c, the light 330 traveling in the EO polymer in theactive region is vertically confined by the sol-gel core 304 and bufferlayer 324. Because the refractive index of the sol-gel core n₂ isgreater than the refractive index n₃ of the side-cladding 306, sol-gelcore also acts as a virtual side-cladding 331 to confine light 330laterally in the overlying EO polymer layer thereby creating awell-defined EO polymer waveguide in the active region; this is theso-called strip loading effect. The modulated light 330 is coupled backinto the sol-gel waveguide defined in sol-gel core 304 throughtransition region 320 and coupled to the output SMF. The difference inrefractive index between the sol-gel core 304 and the sol-gelside-cladding 306 may be at least 0.01 and may preferably be at least0.04-0.06. The greater the contrast between the indices the better thelateral confinement of light in the EO polymer waveguide. However, ifthe difference is too large coupling to the optical fiber will suffer.Without this virtual side-cladding the light would expand laterallywithin the EO polymer layer in the active region and coupling back intothe sol-gel waveguide would be poor; the waveguide would also becomemultimode in the plane. Conversely, because the light is confined withinthis virtual side-cladding without etching the EO polymer layer thelight does not scatter, the waveguide remains single-mode and theinsertion loss is much lower, typically 5 dB or lower.

The strip-loaded design provides excellent confinement of light in theEO polymer waveguide in the active region, which means theinter-electrode separation can be reduced without deleterious opticalcoupling to either electrode. This in turn means that the V_(π) can bereduced. Light 330 is well confined to the sol-gel core 304 in thepassive regions and the EO polymer layer 312 in the active regions.Light 330 does not encounter rough edges as it transitions to and fromthe sol-gel and EO polymer waveguides. This reduces insertion loss.

A particular embodiment of the strip-loaded modulator 300 is depicted inFIG. 16. The structure is the same as that shown in FIG. 14 a butspecific refractive indices are provided. Sol-gel under cladding 302 andsol-gel buffer layer 324 have refractive indices n₁=n₇=1.485. Sol-gelcore 304 has refractive index n₂=1.539. Sol-gel side-cladding 306 (notshown) also has a refractive index n₃=1.485. The EO polymer layer has arefractive index n₄=n₆=1.529 in the passive regions and n₅=1.609 in theactive region.

Light 330 is confined to the sol-gel core 304 in the passive regions andthe EO polymer layer 312 in the active regions. In the active region,where the sol-gel core 304 acts as a bottom cladding for the EO polymerwaveguide, substantially no light penetrates to the sol-gel core. Theside view shown in FIG. 16 illustrates the center region of the sol-gelstraight core 310. Strong mode confinement (>80%) was calculated for theEO polymer core, which is competitive with that of previous hybridmodulators. Both the optical fiber coupling loss and the tapertransition losses were found to be very low, and with resultinginsertion losses in the range of 5 dB or less.

A fabrication sequence for the particular embodiment of the strip-loadedmodulator shown in FIG. 16 is now described. Two sol-gel compositionswere used for this device. A 95/5 molar ratio ofmethacrylpropyltrimethoxysilane (MAPTMS) to zirconium(IV)-n-propoxide(ZPO) was used as the cladding in both the active and passive sectionswith an index of 1.485. A 30/70 molar ratio of MAPTMS todiphenyldimethoxysilane (DPDMS) was used as the core in the passivesection and as the strip-load in the active section with an index of1.539. 1.5 weight % Irgacure 369 photoinitiator was added to make thesol-gel photo-patternable. Both sol-gels were hydrolyzed with a ratio ofwater:methoxy of 0.37 using 0.1 N HCl. The ZPO was chelated with a 1:1molar ratio of methacrylic acid (MAA). The EO polymer used was aguest-host material, 25 weight % AJLS102 chromophore doped intoamorphous polycarbonate, a high glass transition temperature (T_(g))host polymer.

Mach-Zehnder modulators were fabricated as follows: A 6 μm layer of the95/5 sol-gel was spin-cast onto a Ti coated Si wafer and cured for 1hour at 135° C. A 1.3 μm layer of Shipley 1813 photo-resist wasspin-cast onto the sol-gel layer and prebaked for 8 minutes at 110° C.Next, waveguide trenches were patterned in the resist by exposing for 23s in a Carl Zuss MJB3 mask aligner and developing. 3 μm deep trencheswere then wet-etched into the sol-gel at a rate of 0.1 μm per minute byimmersion in a 1:6 buffered oxide etchant. The resist was then strippedand the 30/70 sol-gel core was spin-cast into the trenches, photo-curedfor 30 s, and developed in a 50:50 mixture of acetone and ethanol. Dueto oxygen inhibition in the free radical photo-polymerization of thesol-gel, the 30/70 MAPTMS/DPDMS sol-gel achieved planarization at thetop of the trenches etched in the cladding.

Next, a 1.1 μm layer of EO polymer dissolved in cyclopentanone wasspin-cast and dried under vacuum at 100° C. After drying the refractiveindex tapers were created in the EO polymer using a grayscale mask fromBenchmark Technologies. The mask consists of two 1.5 mm long sections oflinearly varying optical density connected by a wide chrome strip 1.5 cmin length which prevents the active section from becoming bleached. Anexposure dose of 9 kJ/cm² of Hg i-line radiation was applied whichcreated refractive index tapers that can be seen in FIG. 15. After indextapering, the top sol-gel buffer layer (MAPTMS/ZPO 95/5) was deposited.The top electrode was deposited and patterned, after which the polymerwas poled, thereby completing the modulator.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

1. An electro-optic (EO) polymer/sol-gel modulator, comprising: a sol-gel under cladding having a refractive index n₁; a sol-gel core having a refractive index n₂>n₁; a sol-gel side cladding having a refractive index n₃<n₂ in the same plane as the sol-gel core; an EO polymer layer having a graded refractive index, said layer having an index n₄<n₂ in a first passive region that provides a top cladding for the sol-gel core to define a first sol-gel waveguide in said first passive region, said index graded in a first transition region from n₄ up to n₅>n₂ in an active region so that the sol-gel core provides a bottom cladding for the EO polymer layer to define an EO polymer waveguide in the active region, and said index graded in a second transition region from n₅ down to n₆<n₂ in a second passive region that provides a top cladding for the sol-gel core to define a second sol-gel waveguide in the second passive region, said EO polymer layer poled along a direction; a sol-gel buffer layer above the EO polymer layer, said buffer layer having a refractive index n₇<n₅ in the active region; and first and second electrodes on opposite sides of the EO polymer waveguide in the active region, said electrodes configured to receive a voltage signal to apply an electric field along the poling direction to change the refractive index of the EO polymer waveguide in the active region.
 2. The EO polymer/sol-gel modulator of claim 1, wherein the EO polymer layer is of uniform thickness over the sol-gel core and sol-gel side cladding.
 3. The EO polymer/sol-gel modulator of claim 1, wherein the sol-gel core as the bottom cladding defines a virtual side-cladding in the EO polymer layer for the EO polymer waveguide in the active region.
 4. The EO polymer/sol-gel modulator of claim 1, wherein n₃+0.04<=n₂<=n₃+0.06.
 5. The EO polymer/sol-gel modulator of claim 1, wherein said EO polymer layer has a substantially uniform index n4 in the first passive region, a substantially uniform index n5 in the active region and a substantially uniform index n6 in the second passive region.
 6. The EO polymer/sol-gel modulator of claim 5, wherein the difference between n₅ and n₄ or n₆ is at least 0.015.
 7. The EO polymer/sol-gel modulator of claim 5, wherein in said first passive region n₂-n₄ is at least 0.0075, in said active region n₅-n₂ is at least 0.0075 and in said second passive region n₂-n₆ is at least 0.0075.
 8. The EO polymer/sol-gel modulator of claim 1, wherein the insertion loss of the modulator is no greater than 5.5 dB.
 9. The EO polymer/sol-gel modulator of claim 1, wherein an inter-electrode spacing between the first and second electrodes is no more than 10 microns.
 10. The EO polymer/sol-gel modulator of claim 1, wherein the EO polymer layer is poled along a direction approximately perpendicular to the layer, said first and second electrodes beneath the sol-gel under cladding and above the sol-gel buffer layer respectively.
 11. An electro-optic (EO) polymer/sol-gel modulator, comprising: a sol-gel under cladding having a refractive index n₁; a sol-gel core having a uniform refractive index n₂>n₁; a sol-gel side cladding having a refractive index n₃<n₂ in the same plane as the sol-gel core; a uniform thickness EO polymer layer over the sol-gel core and sol-gel side cladding, said layer having a graded refractive index n₄<n₂ in a first passive region that provides a top cladding for the sol-gel core to define a first single-mode sol-gel waveguide in said first passive region to receive light from a first single-mode fiber, said index graded in a first transition region from n₄ up to n₅>n₂ in an active region so that the light travels from the sol-gel core to the EO polymer layer, said sol-gel core providing a bottom cladding and a virtual side cladding for the EO polymer layer to define a single-mode EO polymer waveguide in the active region so that the light traveling there through is confined inside the EO polymer waveguide, said index graded in a second transition region from n₅ down to n₆<n₂ in a second passive region that provides a top cladding for the sol-gel core to define a second sol-gel waveguide in the second passive region so that said light travels from the EO polymer layer to the sol-gel core to output light to a second single-mode fiber, said virtual side cladding laterally confining said light as it travels through the EO polymer waveguide to couple light between the sol-gel core and said EO polymer waveguide, said EO polymer layer poled along a direction; a sol-gel buffer layer above the EO polymer layer, said buffer layer having a refractive index n₇<n₅ in the active region; and first and second electrodes on opposite sides of the EO polymer waveguide in the active region, said electrodes configured to receive a voltage signal to apply an electric field along the poling direction to change the refractive index of the EO polymer waveguide in the active region and modulate the amplitude of the light traveling there through.
 12. The EO polymer/sol-gel modulator of claim 11, wherein said EO polymer layer has a substantially uniform index n₄ in the first passive region, a substantially uniform index n₅ in the active region and a substantially uniform index n6 in the second passive region.
 13. The EO polymer/sol-gel modulator of claim 12, wherein the difference between n₅ and n₄ or n₆ is at least 0.0015.
 14. The EO polymer/sol-gel modulator of claim 12, wherein in said first passive region n₂-n₄ is at least 0.0075, in said active region n₅-n₂ is at least 0.0075 and in said second passive region n₂-n₆ is at least 0.0075.
 15. The EO polymer/sol-gel modulator of claim 12, wherein the insertion loss of the modulator is no greater than 5.5 dB.
 16. The EO polymer/sol-gel modulator of claim 11, wherein n₃+0.04<=n₂<=n₃+0.06. 